JPS6143721B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to function generators for producing an output signal that is a predetermined function of one or more input signals, and more particularly to a function generator for producing an output signal that is a predetermined function of one or more input signals; The present invention relates to a function generator that utilizes a cam-following servo mechanism to create a desired functional relationship.

Pneumatic controllers are known in the art in which the gain of the controller is manually adjusted to produce an output signal that is linearly proportional to the input signal. It also utilizes a pneumatic servomechanism and uses a controller that varies the gain of the controller in response to an input signal, thereby producing an output signal according to a function determined by the servomechanism and the controller input signal. Function generators are also known. An example of such a function generator is disclosed in US Pat. No. 3,404,604.

The conventional function generators listed above utilize a mechanical cam follower that physically engages the contour of a preformed rigid cam member to set the gain of the controller according to the contour of the cam member. It was hot. The cam member was rotatable to provide a differently contoured surface to the cam follower for each different input signal. Thereby the function generator was able to produce an output signal that varied according to a predetermined function of the input signal. Cams with different contours provided different functional relationships. However, these function generators had certain limitations primarily due to the use of mechanical cam followers.

In order for the cam member to provide repeatability and long-life performance, the cam member must be constructed of a rigid metallic material that will not deteriorate due to prolonged moving contact with the mechanical cam follower. Nakatsuta. This means that the cam member must be compressed or otherwise cut with a machine tool.
This made it difficult to form a cam profile of the required function.
Therefore, it has been difficult, if not impossible, to customize the necessary function profile for a particular unit on-site.

Additionally, there are limitations due to having the follower contact the edge of the cam via a roller. First, the roller could not follow the concave cam profile if the radius of curvature of the concave cam profile was smaller than the radius of curvature of the roller. Second, the rollers were unable to follow the step changes in the cam profile.

Further constraints were added by placing the servo summing position in front of the cam member. This arrangement is
Required that equilibrium as the input is increased can only be achieved by configuring the cam member to provide a continuously increasing negative feedback signal. In practice, this prevented the cam from being contoured to a constant radius anywhere, and also prevented the cam profile from reversing its slope.

These and other problems associated with prior art devices are solved by the invention described below.

In accordance with the present invention, an improved function generator utilizes a unique motion balance servomechanism for sensing the edge of a flexible cam member that can be easily fabricated without mechanical contact with the cam member. is provided.

The servomechanism has a movable nozzle assembly;
The assembly establishes a backpressure signal depending on the degree of restriction of the nozzle. This back pressure signal is transmitted to a piston assembly that moves the nozzle. Nozzle restriction is provided by a cam member. The nozzle is also configured to move along the surface of the cam member until the nozzle reaches an edge of the cam member that provides an equilibrium position for relatively unrestricted fluid flow through the nozzle. It will be done.

Since the nozzle has no mechanical contact with the cam member, there is no need for the cam member to be made of a rigid and strong material, e.g. metal, and the cam member can be made in the field from a flexible material, e.g. thin plastic. Can be made of easily cut materials.
In fact, a very flexible material is chosen in order to self-align the cam surface to the height of the nozzle. This is done by simply cutting out the plastic cam member with a knife or scissors to provide the desired cam profile, i.e. the functional relationship between the input and output signals of the function generator installed at the particular site. adjustment or even programming is possible. Also, since the nozzle is very small and oriented substantially perpendicular to the plane of the cam member, there is no minimum dimension constraint on the cow shape that the nozzle can follow. Also, since the nozzle is positioned by a piston rather than by rolling up or down a cam,
There are no restrictions on the slope of the cam, and stepwise changes are possible.

Certain embodiments of the present invention facilitate the creation or adjustment of cam member profiles in the field and also ensure that the flexible cam member is always guided into the proper relationship with the nozzle. This is guaranteed. To do so, a nozzle assembly is formed. The nozzle assembly has a substantially vertically oriented nozzle outlet port on one side of the cam member and a spaced apart guide member formed on the other side of the cam member. This arrangement of the nozzle and guide member prevents the cam member from separating from the nozzle due to impact, while allowing the cam member to move freely between the nozzle and the guide member. The nozzle and the guide member also both have sloped surfaces converging towards the cam member to guide the edges of the cam member into the gap provided between the nozzle and the guide member. To facilitate easy programming of the cam member, the guide member has an opening formed in alignment with the exit port of the nozzle. By inserting a marking pencil into this opening, one side of the uncut cam member blank is easily marked in relation to the nozzle position relative to the complete cam member. Since the markings on the cam member are aligned with the nozzle exit ports, the cam member can be easily marked at each increment of the desired functional relationship of the input signal to the output signal, and then cut along the contour defined by the increment markings to provide a programmed program cam member.

From the foregoing it can be seen that one object of the present invention is to provide a servomechanism for a function generator that accurately follows complex functional contours, such as those with steps and inverted slopes. Dew.

Another object of the present invention is to provide a servomechanism for a function generator that accurately follows the contour of a cam member without making physical contact with the cam member.

Yet another object of the present invention is to provide a flexible cam member for a function generator that is easily programmed and created in the field to provide the desired function relationship for a particular function generator. It is.

These and other objects and features of the present invention will become clearer from the following description.

Next, the drawings will be described, but it should be understood that the drawings are created to explain a preferred embodiment of the present invention, and the present invention is not limited thereto. Probably.

Referring specifically to FIG. 1, a function generator assembly 10 powered by a main power supply S is illustrated. Function generator 10 is connected to controller assembly 12
, and the gain arm 14 of the controller assembly 12 receives the output signal from the controller assembly 12.
is automatically adjusted by gain servomechanism assembly 16 to provide E ₀ . The output signal E ₀ is a function of the input signal E ₃ and of the input signals E ₁ and _{E 2 multiplied} by a set amount G of the gain arm 14, which is set as a function of the input signal E 4 .

Controller assembly 12 may be any conventional electrical or pneumatic controller. Control assembly 12 compares input signals E ₁ and E ₂ at summing location 18 which provides a signal along line 22 to calculation unit 20 representing the difference between signals E ₁ and E ₂ . do. usually,
One of the signals E ₁ and E ₂ is the reference signal and the other signal is the variable signal to be measured. Calculation section 20
multiplies the received difference or error signal by a gain G determined by the setting of gain arm 14 and sends the multiplied signal along line 24 to a second summing location 26 . The second addition position 26 is line 24
The signal transmitted from the output signal E ₃ is compared with the input signal E 3 and with the feedback signal derived from the output signal E ₀ and received from the feedback loop 28. Feedback loop 28 repositions the vanes and nozzles (not shown) of controller assembly 12, and also differentiates and differentiates by means of restriction and volume appropriately placed in feedback loop 28, as is well known in the art. Performs integral control operation. Second
The output of summing position 26 is transmitted to transmission section 30 along line 32 establishing output signal E ₀ .

The gain servo mechanism assembly 16 is the gain arm 1
4 and thus set the gain G of controller assembly 12 in response to input signal E ₄ . For this purpose, a signal E ₄ is transmitted to the cam actuating member 34 . The cam actuating member 34 rotates the cam 36 by a predetermined angle determined by the level of the input signal _E4 . Summing position 38 relates the radius of the set angular position of cam 36 to the position of cam follower 40 and provides a signal along line 42 to cam follower actuation member 44 when the position of follower 40 does not properly correspond to the cam radius. provide. The actuating member 44 is
When a signal is received along line 42, follower 40 is driven relative to the radius of cam 36 until a suitable corresponding position relative to the radius of cam 36 is obtained by follower 40. The summing position 38 is then in equilibrium and the follower 40 remains stationary until a new position of the cam 36 is produced by a different input signal _E4 . Follower 40 is coupled to gain arm 14 and causes gain arm 14 to move when follower 40 is moved by actuation member 44 . Therefore, the gain G of controller assembly 12 is varied according to the level of input signal _E4 .

Referring now to FIG. 2, air controller assembly 1
2 are the first and second addition positions 18 and 2 in FIG.
A pair of beams 46 and 48 are provided with pivot points 50 and 52 that function as 6. Input signals E ₁ and E ₂ are connected to beams 56 on either side of pivot point 50.
The signals E ₁ and E ₂ cause the beam 46 to swing in response to the pressure differential provided to the bellows A and B attached to the bellows A and B by the signals E 1 and E 2 . Thus, if one of the input signals E ₁ , E ₂ is the variable being measured and the other is a reference set point, the deflection of the beam 46 is indicative of the error signal therebetween.
The deflection of beam 46 is transmitted by linkage assembly 58. The linkage assembly 58 corresponds to the line 22 in FIG. 1 connecting the first summing position 18 to the calculator 20 in FIG. Computing section 20 of FIG. 1 is formed as a vane and nozzle assembly 60, 62 connected to a sector arm 64 which functions as the gain arm of FIG. Sector arm 64 is connected to vane 60 via a common connection point 66 that connects proportional link 68 to vane 60 . Vanes 60 thus respond to movement of sector arm 64. The deflection of beam 46 displaces vane 60 through linkage assembly 58 relative to nozzle 62, creating a new nozzle 62 backpressure condition and creating an imbalance in the beam that must be rebalanced. This rebalancing effect occurs at the pivot point 5 of the beam 48.
This is done by bellows C and D disposed at both ends of 2. C bellows is connected to signal E ₃ ;
The pressure must be balanced by the bellows D. The bellows is fed back and output
Connected to E ₀ . E ₀ is proportional to the back pressure in nozzle 62 increased by booster 70 .

The spacing between vanes 60 and nozzles 62 initially set by sector arm 64 is the new input signal.
When equilibrium is broken by the deflection of beam 46 in response to E ₁ and E ₂ , the backpressure signal established along line 72 by nozzle 62 to booster 70 also changes. Booster 70 provides a new amplified output signal E ₀ along line 74 that is proportional to the new backpressure signal. The new output signal E ₀ is coupled in feedback to the bellows D by line 76. When the new output signal E ₀ reaches the bellows D, the beam 48 is deflected with the nozzle 62 attached thereto to return to the initial vane 60 and nozzle 62 spacing under the new output E ₀ condition. It will be done.

Differential control action may be provided to the controller 12 by delaying application of the rebalancing output signal E ₀ to the bellows D. This is accomplished by placing a restrictor 78 on the feedback line 76 connecting the output _E0 to the bellows D. Similarly, as is well known, the integral control operation is
The controller 12 may be provided by placing a restrictor 80 and a volume 82 in series between the booster 70 and the C bellows and in the line connecting them.

The controller 12 outputs E ₀ as a function of the input signal increased by the controller's set gain G.
, the controller cannot itself produce a nonlinear output E ₀ in response to a linear input, or vice versa. To enable such a linear to non-linear input-output relationship, a gain servomechanism 16 is utilized to control the controller 12 according to the setting of the sector arm 64 and thus the level of the input signal _E4 . To automatically vary gain G.

The gain servo mechanism 16 is a motion-balanced servo mechanism equipped with a servo nozzle 86, which follows the edge of the cam 36 and is balanced along the edge to be stationary. This coupling relationship between cam 36 and nozzle 86 acts as addition position 38 in FIG. As the nozzle 86 is moved from the edge of the cam 36 due to the new cam 36 position, the nozzle 86 backpressure signal changes and the piston assembly 88
transmitted to. The piston assembly 88 connects the actuating nozzle 86 to the cam 36 as the actuating member 44 of FIG.
Drive towards the edge of. Since the nozzle 86 is attached to the sector arm 64, moving the nozzle 86 back to the edge of the cam 36 will move the sector arm 64 back, changing the gain G of the controller as previously discussed. Therefore, it can be seen that rotating the cam 36 moves the nozzle 86 to a different edge position of the cam 36, causing the controller 12 to set a different gain G.

Cam 36 rotates in response to signal _E4 . To do this, bellows L is mounted between rigid surface 60 and pivoting bellows beam 92 to rotate beam 92 about its pivot point in response to the pressure of input signal _E4 . Beam 92 is connected to cam 36 by a connecting link 94. The connecting link 94 responds to the movement of the beam 92 by connecting the cam 3.
Rotate 6. As best seen with reference to FIG. 5, the connecting link 94 includes a main rigid connecting member 95 and a flexible retaining member 97. Flexible member 97
is mounted in an arcuate manner on the rigid member 95 and holds the rigid member 95 between the beam 92 and the cam 36;
Any free movement between beam 92 and cam 36 is prevented.

As best seen with particular reference to FIGS. 3 and 4, the cam 36 is made of a thin flexible plastic material that can be easily cut to the desired edge profile in the field, as described below. made from materials. Applicants have found that a 0.007 inch (0.178 mm) thick Mylar polyester film is a suitable material for the cam 36. The cam 36 is mounted between a pair of plates 96 and 98, which are secured together by a knurled nut 100. The assembly is adapted to pivot about pivot 102. Connecting link 94
is connected to a point 104 on plate 98 offset from pivot 102 to rotate cam 36 in response to the linear movement of connecting link 94.

Because cam 36 is flexible, nozzle 86 is attached to sector arm 64 via nozzle mounting and cam guide assembly 106. The assembly ensures that the cam 36 is directed toward the nozzle 86 without bending or folding when a corner is encountered. Guide assembly 106
is formed as a U-shaped bracket with upper and lower legs 108 and 110 between which the cam 36 is guided by sloping edges 112 and 114 formed in the cam guide 145 and the nozzle 86, respectively. be done. Edges 112 and 114 are approximately 45°~
It is angled at 60 degrees to easily guide the cam 36 toward the nozzle 86 when the cam 36 momentarily disengages from the nozzle support assembly 106. Lower leg 11
0 installs the nozzle 86 there and directs the air to the cam 36.
discharge perpendicular to the surface of the This vertical alignment ensures that the airflow from nozzle 86 is always directed toward cam 36.
is guaranteed to collide with the edge at a known 90°.
Upper leg 108 has cam guide 145 mounted thereon. An opening 116 passes through the guide 145 and connects the nozzle 8.
6 and is formed in a straight line. This opening 116 marks the uncut cam 36 and provides inputs E ₁ , E ₂ , E ₃ , E ₄
and the output E ₀ to program and create the cam to provide the desired functional relationship between the output E 0 and the output E 0 . Aperture 116 is a circular hole approximately 0.161 inch (4.089 mm) in diameter, with a smaller 0.047 inch (1.194 mm) diameter hole near cam 36. This allows a fine-tipped felt nib to be easily and accurately inserted into the hole 116 to mark the cam and to create and modify the cam 36.

Blank cam 36 has flexible hose 11 connecting piston assembly 88 to back pressure from nozzle 86.
8 and reconnecting the piston assembly 88 to an external adjustable pressure source, such as a manual regulator. hose 118
are separated to prevent the piston assembly from driving the nozzle means 86 to an edge of the cam 36 that is not programmed. Since the input to piston assembly 88 is externally generated, no feedback from nozzle 86 is actuated and nozzle 86 is connected to cam 3.
It can be located anywhere in the radius of the cam 36 rather than just at the edge of the cam 36. When taking a diagram of the desired functional relationship between the inputs E ₁ , E ₂ , E ₃ , E ₄ and the output E ₀ , the inputs E ₁ , E ₂ , E ₃ , E ₄ are each represented by 10
% increments, the manual regulator controls the nozzle to the stationary cam 36 until a suitable gain G is achieved by the controller to provide the desired output signal _E0 .
adjusted to move along the surface of the The cam 36 can be made as described above by inserting the tip of a felt pen through the opening 116 and marking the cam.
The amount increased by % is entered. The marked cam is then removed from between plates 96 and 98 by loosening knurled nut 100 and sliding it from between plates 96 and cam hub plate 98. The progressively increasing points marked on the cam are then connected in a smooth curve through each of these points. The thin flexible cam 36 is simply cut with scissors along the drawn curve, resulting in an edge of the cam 36 that defines the marked curve. Cam 36 is then repositioned between plates 96 and 98;
A connecting hose 118 is also connected between piston assembly 88 and nozzle 86 . Since the nozzle 86 will itself be positioned along the programmed edge of the cam 36, the programmed edge of the cam 36 will cause the appropriate relationship according to the previous diagram to be applied to the input signal.
It will be maintained between E ₁ , E ₂ , E ₃ , E ₄ and the output signal E ₀ .

The operation of programmed function generator 10 is as follows. As new input signals E ₁ , E ₂ , E ₃ , E ₄ are applied to function generator 10, cam 36 will rotate to a new position either releasing or restricting nozzle 86. If nozzle 86 is edge-positioned, it will affect the amount of air escaping from nozzle 86. Nozzle 86 and piston assembly 88 are mounted in a common volume chamber 122 in the base of controller 12 so that changes in back pressure in nozzle 86 are transmitted to piston assembly 88. The volume chamber 122 has an air flow rate of approximately 0.08 scfm (0.00038 ft ³ /
min standard condition) and supplies both the nozzle 86 and the piston assembly 88 along conduit 118. Volume room 1
22 is connected to piston assembly 88 through a 0.018 inch (0.457 mm) orifice 123 located at the end of cylinder 124 of piston assembly 88. This orifice 123 serves to sufficiently slow the response of the cylinder assembly 88 to changes in nozzle back pressure to prevent overshoot and to stabilize the servomechanism 16.

Certain features of the present structure may provide additional benefits. The cylinder 124 of the piston assembly 88 is formed from precision bore Pyrex tubing with an inside diameter of 10,000 inches (25,000 mm).
A carbon piston 126 having an inner diameter of 0.9993 inch (25.3822 mm) and a length of 1 inch (25.4 mm) is installed within the cylinder 124 . The diametric gap between tube 124 and piston 126 allows piston 126 to move freely without allowing air to escape past piston 126. Typical cylinder 124 operating pressure is 4.0 psi
(0.28Kg/cm ² ) (retracted) to 8.3psi (0.58
Kg/cm ² ) (fully extended state). These pressures are determined by a cylinder return spring 134 made of #302 stainless spring tempered wire and wrapped around the cylinder 124. Cylinder 124 is mounted to base 90 and is held airtight thereto by a Buna NO-ring, not shown. The O-ring not only seals the pressure in cylinder 124, but also seals the pressure in cylinder 124.
It extends slightly beyond the end of the base of 24 and provides a cushion or stop for the piston to minimize overtravel. Clear polyolefin heat shrink tubing 128 is heat shrunk onto the cylinder 124 to partially cover the end of the cylinder 124 to not only retain the piston 126 but also provide maximum overtravel stopping means to ensure that the piston 126 is Avoid popping out of the cylinder 124 under back pressure conditions. Clear heat shrink tubing 128 is used to allow inspection of the O-ring, piston 126 and cylinder bore without disassembly. carbon piston 12
6. The assembly of the glass cylinder 124 is advantageous, for example, in terms of a low coefficient of friction, as well as a coefficient of thermal expansion. Only 0.0001 in a temperature range of 100〓 (55.6℃)
There is only an inch (0.0025mm) difference in elongation. Break away △P is only 0.1psi
(0.007Kg/cm ² ).

Connecting rod 130 transmits the movement of piston 126 to sector team 64 by means of its coupling. The connecting rod 130 is not attached to the piston 126 at one end and is connected to the piston 12 by a deep countersunk hole 132 formed in the face of the piston 126.
You will be guided to the center of 6. The connecting rod 130 is held against the piston 126 by a return spring 134, as will be described below. The other end of the connecting rod 130 is provided with a conically shaped hole 136 that fits into a hemispherical projection 138 on the nozzle support assembly 106. The two parts are loosely preloaded together and held by screws 140 and springs. A conical hole 136 and a preload would otherwise exist between the connecting rod and the nozzle support assembly 106, and if allowed to exist would result in instability (hunting).
required to eliminate stray movements that cause
The hemispherical surface 138 also provides a ball and socket joint that can be dealigned without mating.

The installation method of the spring 134 will be explained as follows.
One end of the cylinder return spring 134 is connected to the connecting rod 13
0, and the other end is bolted to the main body 90 with a screw 144. This places the connecting rod 130 in compression between the spring load at one end and the force of the piston 126 at the other end. Almost all of the force on piston 126 is transferred from spring 134 to connecting rod 130 to piston 12.
6, air is constrained by the loop defined by base 90, spring screw 144, and back to spring 134. Therefore, only a small component of this load is applied to the sector arm 64.
, where this load causes the sector arm 64 to
swings, causing a null shift. The design of the connecting rod is such that at the free end of the spring 134,
Note that the angular misalignment can be large, which is normal for springs with a large ratio of outer diameter to wire diameter.

Referring now to the construction and mounting of nozzle 86 in detail, nozzle 86 is secured to nozzle support assembly 106 by screws 146 and is configured to compensate for cam height variations caused by tolerance stack-up. It is said that it can be adjusted to The nozzle 86 is 0.170 inches high from 45° to 60° to guide the cam 36 into the nozzle face even when adjusted upwardly to compensate for stacking of the cam height tolerance chamber.
It has a chamfered part of °. Therefore, the edge of the cam 36 will not interfere with anything under any circumstances, but will be guided into the proper position in the face of the nozzle 86. The surface of the nozzle 86 is a gently convex cone. There is a 0.0220 inch (0.5588 mm) diameter nozzle exit in the center of the face. The conical surface causes the plane of the cam 36 to be slightly out of alignment with the axis of the nozzle 86. The face of nozzle 86 defines a region of high velocity, low pressure air that escapes from the outlet of nozzle 86 such that cam 36 is drawn toward the face of nozzle 86 . However, the cam is not normally drawn into the face of the nozzle 86, but rather rests on an approximately 0.001 inch (0.025 mm) film of air over the center of the face of the nozzle 86. Nozzle 86
The cam guide 145 is in line with the cam 36 and on the other side of the cam 36.
The cam guide 145 serves to guide the cam 36 within 0.015 inch (0.381 mm) of the end of the nozzle 86 after the cam 36 is drawn toward the nozzle 86 by the escaping air. . The cam guide 145 normally does not operate in contact with the cam 36;
It is threaded down to within 0.005 inches (0.127 mm) to prevent the cam from being thrown off the nozzle 86 by severe mechanical shock.

From the foregoing, using an easily programmed cam and an accurate non-contact cam follower,
It will be appreciated that an improved function generator has been provided which produces more accurate and adaptive units.

Other improvements and modifications will occur to those skilled in the art upon reading this specification. Obviously, the basic concept described here could also be applied very easily to electrical function generators. Accordingly, the aforementioned improvements and modifications have been omitted for ease of explanation and understanding, and it is to be understood that these various aspects are also within the scope of the present invention.

[Brief explanation of the drawing]

FIG. 1 is a functional block diagram of a function generator according to the invention. FIG. 2 is a schematic diagram of the function generator functionally represented in FIG. FIG. 3 is a plan view of the actual function generator schematically represented in FIG. FIG. 4 is a side view of the function generator of FIG. 3. FIG. 5 is an enlarged view of the connecting member connecting the cam member to the input bellows in the function generator of FIGS. 3 and 4. FIG. The main parts in the figure are as follows. 10: Function generator, 12: Controller assembly, 14: Gain arm, 16: Gain servomechanism assembly, 18: Addition position, 20: Calculation unit, 26: Second addition position, 2
8: Feedback loop, 30: Transmission section, 3
4: cam operating member, 36: cam, 38: addition position, 40: cam follower, 44: cam follower operating member, 58: link device assembly, 60: vane,
62: nozzle, 64: sector arm, 70: booster, 86: servo nozzle, 88: piston assembly, 94: connecting link, 95: main rigid connecting member,
97: Flexible holding member, 106: Nozzle mounting and cam guide assembly, 124: Glass cylinder, 12
6: Carbon piston, 130: Connection rod, 1
28: Heat shrink tube, 134: Spring, 145: Cam guide, E ₁ , E ₂ , E ₃ , E ₄ : Input signal, E ₀ : Output signal.

Claims (1)

[Claims] 1. providing a movable nozzle means for providing various back pressure signals depending on the degree of restriction imposed on the fluid flow therethrough; having a cam member mounted substantially perpendicularly to said nozzle means; restricting means for restricting fluid flow through the nozzle means along a plane of the cam member; at an edge of the cam member where fluid flow through the nozzle means is substantially unrestricted; a movable gain adjustment arm coupled to the nozzle means and adapted to move in response to movement of the nozzle means; and a pneumatic controller adapted to provide an output signal as a function of an input signal and the position of the gain adjustment arm, and further configured to move the cam member relative to the nozzle means in response to the input signal of the controller. A coupling means is provided for movement, the coupling means comprising a rotatable pivot mounted to and adapted to rotate the cam member, a bellows and a pivotable pivot movable in response to expansion of the bellows. a bellows assembly having an arm member responsive to an input signal of the controller; a rigid connecting bar having an opening at each end for connecting the pivot to the arm member of the bellows assembly; a flexible connecting bar having an opening at each end spaced further apart than an opening in the rigid connecting bar, the openings in the rigid bar and the flexible bar connecting the pivot to the bellows assembly; A function generator characterized in that the flexible bar is bent to align when placed over the rigid bar to retain the rigid bar when coupled to a three-dimensional arm member. .